1. Field of the Invention
The present invention relates to a surface light source device of side light type. The present invention is applied to, for example, a back lighting of a liquid crystal display.
2. Related Art
A surface light source device of side light type has been applied to, for example, a liquid crystal display and illuminates the liquid crystal display from its back. This disposition is suitable for a thin configuration of the device.
Usually a rod-shape light source such as a cold cathode tube is employed as a primary light source with respect to the surface light source device of side light type, wherein this rod-shape light source is disposed sidewardly of a guide plate (a plate-like light guiding member). Illumination light that has been emitted from the primary light source is introduced into the guide plate by passing through a side end surface thereof The introduced illumination light propagates through the guide plate and, in this process, light emission occurs from a major surface of the guide plate toward a liquid crystal panel.
As guide plates employable in the surface light source of side light type, there are known two types of guide plates, one of which is a type having generally uniform thickness and the other of which is a type having a tendency that plate thickness decreases as being distant from the primary light source increases. In general, the latter emits illumination light more efficiently than the former.
FIG. 10 is an exploded perspective view illustrating a surface light source device of side light type that uses the latter type of guide plate. In FIG. 11, illustration is made of a section taken along a line (A--A) in FIG. 10. Referring to FIGS. 10 and 11, a light source device 1 is provided with a guide plate 2, a primary light source 3 disposed sidewardly thereof, a reflection sheet 4, a light diffusion sheet H, and a prism sheet 5 providing a light control member. The reflection sheet 4, guide plate 2, light diffusion sheet H and prism sheet 5 are laminatedly arranged.
The guide plate 2 is a transparent guide plate 2 that has a wedge-shaped cross section and is a transparent member formed of acrylic resin. Usually a light diffusion surface is formed on a back surface 2B of the guide plate 2. In some cases, as the guide plate 2 there is employed a scattering light guiding plate that consists of a scattering light guiding member. The scattering light guiding member consists, for example, of a matrix that consists of PMMA (polymethyl methacrylate) and a large number of light-transmitting fine particles uniformly dispersed therein. The refractive index of these fine particles differs from that of the matrix.
The primary light source 3 is provided with a cold cathode tube (fluorescent lamp) 7 and a reflector 8 that is disposed at the back of and that is generally semi-circular in cross section. Illumination light is supplied toward the side end surface of the guide plate 2 by passing through the opening of the reflector 8. The reflection sheet 4 is a sheet-like regular reflection member consisting of a metal foil or the like, or a sheet-like irregular reflection member consisting of a white PET film or the like.
Illumination light L from the primary light source 3 is introduced into the guide plate 2 by passing through an incidence surface 2A that is one side end surface of the guide plate 2. Illumination light L propagates toward a distal end of the guide plate 2 while being repeatedly reflected between the back surface 2B disposed along the reflection sheet 4 and an emission surface 2C. During this propagation, illumination light L is subject to scattering action of the back surface 2B having light diffusion property. If the reflection sheet 4 is an irregular reflection member, illumination light L is subject to also the resulting irregular reflection action.
Illumination light L has gradually decreased its angle of incidence upon the emission surface 2C each time the repetition thereof made by the back surface 2B increases. Such decrease in angle of incidence causes an increase in components whose angles become below the critical angle with respect to the emission surface, thereby promoting the emission of the resulting light rays from the emission surface. As a result, the deficiency in the emission light in a region remote from the primary light source 3 is prevented.
Illumination light that has been emitted from the emission surface 2C has experienced light diffusion made by the back surface 2B having light diffusion property or the irregular reflection made by the reflection sheet 4 and therefore has property of scattering light. However, the main propagation direction of illumination light that has been emitted from the guide plate 2 is inclined toward a distal end (on a side opposite to that where the primary light source 3 exists) with respect to the frontal direction. Namely, the emission light from the guide plate 2 has directivity. Such a property of the guide plate is called "emission directivity".
The prism sheet 5 is disposed in order to correct emission directivity of the guide plate 2. The light diffusion sheet H weakly diffuses illumination light that has been emitted from the guide plate 2 to thereby prevent the light diffusion surface of the back surface 2B from being visually recognized from over the emission surface 2C. The light diffusion sheet H is disposed, as required, further in order to make less prominent brightness, shadows, etc. of portions in the guide plate 2 that are lightened by illumination light. In a case where the guide plate 2 is a scattering guide plate, the light diffusion sheet H is not employed in many cases.
The prism sheet 5 is formed of a light-transmitting sheet material such as polycarbonate. Its prism surface is formed on a surface (outside surface) opposite with the guide plate 2. The prism surface has a large number of projections each of which extends generally in parallel with the incidence surface 2A. Each projection has a pair of slopes that constitute an isosceles-triangular cross section. Angles (angles of inclination) defined between these slopes and the emission surface 2C are equal to each other. Such a prism sheet is called "symmetrical prism sheet".
In a plane perpendicular to the incidence surface 2A the main emission direction of the emission light rays is corrected to the frontal direction of the emission surface 2C. A so-called "double-faced prism sheet" whose both surfaces are each constituted by a prism surface may be used.
Generally, the above-described surface light source device of side light type which uses such a wedge-shaped guide plate and such prism sheet emits a generally uniform output light highly efficiently in the frontal direction.
A structure wherein the emission surface of the guide plate provides a light control surface as indicated by a broken line in FIG. 11 may be employed. This light control surface consists of a large number of projections each of which extends in a direction generally perpendicular to the incidence surface, thereby enhancing outputting efficiency of illumination light.
However, according to the combination with a said conventional prism sheet, output light is hardly directed to the frontal direction of the emission surface 2C correctly. If a small angle of inclination is given to paired slopes forming the projection of the prism surface of the prism sheet, an increase in output light to the frontal direction will be expected but another problem arises.
Namely, difference between the respective output light from the both slopes constituting the slope pair is increased. As a result, fine pulsations in brightness level occur in a direction of repeating of the projections. This pulsation leads to an undesired periodic non-uniformity of brightness. If this device is combined with a unit of a structure having another periodicity, e.g., liquid crystal display panel, such periodic non-uniformity of brightness is likely to cause the occurrence of a moire fringe pattern. Particularly, in a case where the pitch at which picture elements of the liquid crystal panel are arranged and the pitch at which the projections are repeated are approximately equal to each other, tendency that a striking moire fringe pattern will be observed. The moire fringe pattern remarkably degrades the display quality of the relevant display panel.
Also, when having viewed from over the emission surface, the hue of the reflection sheet disposed along the back surface is observed through the slopes where the amount of emission light is less. This is an undesirable level of phenomenon.